Preparation method for the production of active and soluble proteins in prokaryotes and polycistronic vectors therefor

ABSTRACT

Disclosed are a method of producing a target protein in a biologically-active, soluble form in prokaryotic cells and polycistronic vectors therefor.

This application is National Stage entry of PCT/KR04/01393, filed Jun. 11, 2004, which claims foreign priority to Korean application No. 10-2002-0031977, filed May 6, 2004.

TECHNICAL FIELD

The present invention relates to a method of producing a target protein in biologically-active, soluble forms in prokaryotes, and polycistronic vectors therefor.

BACKGROUND ART

The production system of heterogeneous proteins using recombinant E. coli ensures rapid cellular growth rates and high density cultures using inexpensive substrates and uses relatively well-identified genes in comparison with cases using other organisms, thereby making it possible to design various vector systems for facilitating the high expression and purification of heterogeneous proteins (Jeffrey G. T. and Amanda A. et al, (1997), Applied Biochemistry and Biotechnology 66, 197-238).

However, when E. coli is used as a host cell for the production of eukaryotic proteins, E. coli cannot perform post-translational modification such as glycosylation because it does not possess intracellular factors required for protein maturation. In addition, when a heterogeneous protein is expressed in high levels, it is often accumulated in the form of inclusion bodies, which are insoluble precipitates.

Inclusion bodies are typically formed by interaction between hydrophobic surfaces of folding intermediates of a target protein due to imbalance between the production rate and the folding rate of the target protein. In this case, inclusion bodies may be easily isolated, be typically less affected by proteinases and be accumulated in high concentrations in cells, thereby securing high yields and easy isolation of a target protein. Due to these advantages, the strategy of expressing a protein as inclusion bodies is utilized in the production of proteins unfavorable for in vivo folding. However, a target protein expressed as inclusion bodies requires an additional refolding process to recover its biological activity. The refolding of a target protein to an active form is dependent on experience, and is thus always not successful and makes it difficult to scale up the production of recombinant proteins in industrial scales. In addition, high molecular weight antibody proteins, tissue plasminogen activator (tPA) and factor VIII are very difficult to produce in active forms by a refolding process.

As described above, since inclusion body proteins should be refolded to have their structure and biological activity intact (Andrew D. Guise, Shauna M. West, and Julian B. Chaudhuri (1996), Molecular Biotechnology 6, 53-64), a target protein is expressed as a soluble protein using the so-called “in vivo protein folding technique” to induce its correct three-dimensional structure formation in vivo. Since this technique improves problems caused when a heterogeneous protein is expressed as inclusion bodies, it has an industrial importance in producing heterogeneous proteins in E. coli.

The following three strategies are typically used for in vivo folding of proteins.

The first strategy involves the control of protein expression sites and culture environments. When a target protein is designed to be expressed in the cytoplasm, although the target protein is harmful to cells, the cells are not damaged, and the protein is mostly expressed in very high levels. Also, this method facilities the preparation of expression vectors. As another method, the secretion of a target protein to the periplasm has advantages of simplifying protein purification and, compared to the method of expressing a protein in the cytoplasm, reducing protein degradation by proteinases and making disulfide bonding possible to some degree due to a relatively oxidative environment. The advantages further include that an authentic protein can be obtained by removing an N-terminal secretory signal. However, a secreted protein may be aggregated, resulting in formation of inclusion bodies, and reduced folding may occur. In a further method, the secretion of a target protein to culture media may solve the problems associated with protein folding and degradation by proteinases. However, E. coli rarely secretes proteins to culture media, and, even when proteins are secreted to media, proteins are greatly diluted, thus making purification rather difficult. This method is effective only in particular proteins and is thus not a generalized method to prevent inclusion bodies from being formed. Also, the fermentation control is frequently used to increase a soluble protein, and, in most cases, is the most economical method (Korean Pat. Application No. 1997-50023). The reduction of culture temperature is not applied to all proteins, but is a very effective method in many cases because it typically leads to decrease the production rate of a protein below the folding rate of the protein, resulting in no accumulation of folding intermediates with strong aggregation to each other (Schein, C. H. and M. H. M. Noteborn (1988), Biotechnology 6, 291-294; More, J. T., Uppal, F. Maley and G. F. Maley (1993), Protein. Expr. Purif. 4, 160-163).

The second strategy involves the co-expression of chaperones and protein foldases. The chaperones refer to proteins that function to help formation of the desired three-dimensional structure of protein and prevent unnecessary intermolecular or intramolecular interactions. Chaperone proteins derived from E. coli include GroEL, GroES, DnaK, HtpG, SecB and PapD, which protect folding intermediates and prevent aggregation and precipitation, and all of the E. coli chaperone proteins except for PapD (present in the periplasmic membrane) are present in the cytoplasm (Korean Pat. Application No. 2003-7008657; Hartl, F. U., R. Holdan and T. Langer (1994), Trends Biochem. Sci. 19, 20-25; Bernadea-Clark, E. and G. Georgiou (1994), American Chem. Soc. Symp. Ser. Vol. 470, ACS). Foldases refer to an auxiliary protein family that serves to facilitate covalent boding or isomerization during folding. Enzymes stimulating the disulfide bond formation of proteins include DsbA, DsbB, DsbC and DsbD (Creighton, T. E., A. Zapun and N. J. Darby (1995), TIBTECH. 13, 18-27; Gottesman, M. E. and W. A. Hendrickson (2000. Curr. Opin. Microbiol. 3, 197-202).

The third strategy involves the use of fusion proteins. Many proteins have been developed as fusion proteins, which include glutathione-S-transferase, maltose-binding protein, Protein A, tumor necrosis factor-α and lysyl-tRNA synthetase (Smith, D. B. and Johnson, K. S. (1988), Gene 67, 31-40.; Bedouelle, H. and Duplay, P. (1988), Euro. J. Biochem. 171, 541-549.; Nisson, B. et al. (1987), Prot. Eng. 1, 107-113; Korean Pat. Application No. 1996-44010). Also, as described in U.S. Pat. No. 6,027,888, a soluble eukaryotic protein having disulfide bonds can be produced by being expressed in a fused form with disulfide isomerase. In addition, as described in Korean Pat. Application No. 2002-0040497, an H-chain human ferritin protein can be produced as a soluble fusion protein with a L-chain human ferritin protein that is expressed in an insoluble form in E. coli. As described above, various attempts were made to express heterogeneous proteins in soluble fusion protein forms. However, the fusion effect varies according to the type of fusion proteins, as follows: fusion proteins are expressed as inclusion bodies; only a portion of them are expressed as soluble forms; and a protein fused with a target protein functions to aid the folding of the target protein (Savvas C. Makrides (1996), Microbiological Review, 512-538).

Thus, there is an urgent need for techniques allowing the high level production of biologically-active, soluble recombinant proteins in high efficiency and high concentrations.

DISCLOSURE OF THE INVENTION

Based on the above background, the present inventors intended to develop a novel vector system capable of producing in high levels a heterogeneous protein expressed as a biologically active form (but not inclusion bodies) in prokaryotes instead of finding fusion proteins useful for producing proteins by recombinant DNA technology.

As a result, the present inventors found that an expression vector system based on the polycistronic expression of a gene encoding a target protein and a beta-lactamase gene highly expresses both the target protein and beta-lactamase in prokaryotes, resulting in the expression of the target protein in a soluble form in a higher percentage, and is thus effective in the mass production of proteins. Using the established protein expression system, the present inventors developed a method of mass-producing a target protein in a biologically active form, thereby leading to the present invention.

It is therefore an object of the present invention to provide a method of producing a target protein expressed as a biologically active, soluble form instead of inclusion bodies, in prokaryotic cells.

It is another object of the present invention to provide a polycistronic vector system for producing the above target protein in a biologically active form.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram for a process of preparing an expression vector carrying a human growth hormone gene, pTT191;

FIG. 2 is a photograph showing a result of analysis for human growth hormone expression on a SDS-PAGE gel after E. coli BL21 (DE3) was transformed with a pTT191 expression vector (lane 1: protein size marker; lane 2: standard of human growth hormone; lane 3: supernatant obtained by disrupting the IPTG-induced transformant; and lane 4: pellet obtained by disrupting the IPTG-induced transformant);

FIG. 3 is a diagram for a process of preparing pT0191 and pT0-CSF expression vectors respectively carrying a human growth hormone (hGH) gene and a human granulocyte-colony stimulating factor (G-CSF) gene;

FIG. 4 is a photograph showing a result of analysis for human growth hormone expression on a SDS-PAGE gel after E. coli BL21 (DE3) was transformed with a pT0191 expression vector (lane 1: protein size marker; lane 2: standard of human growth hormone; lane 3: whole proteins obtained by disrupting the IPTG-induced transformant; lane 4: supernatant obtained by disrupting the IPTG-induced transformant; and lane 5: pellet obtained by disrupting the IPTG-induced transformant);

FIG. 5 is a construct of a pTR0191 prepared by inserting a human growth hormone (hGH) gene into an expression vector carrying a beta-lactamase gene in a reverse-orientation to the hGH gene;

FIG. 6 is a photograph showing a result of analysis for human growth hormone expression on a SDS-PAGE gel after E. coli BL21 (DE3) was transformed with a pTR0191 expression vector (lane 1: protein size marker; lane 2: standard of human growth hormone; lanes 3 and 4: pellets obtained by disrupting the IPTG-induced transformant; and lane 5 and 6: supernatants obtained by disrupting the IPTG-induced transformant);

FIG. 7 is a photograph showing a result of analysis for human G-CSF expression on a SDS-PAGE gel after E. coli BL21Star(DE3)pLysS was transformed with a pT0-CSF expression vector (lane 1: protein size marker; lane 2: supernatant obtained by disrupting the transformant before IPTG induction; lanes 3 and 4: supernatants obtained by disrupting the IPTG-induced transformant; and lane 5 and 6: pellets obtained by disrupting the IPTG-induced transformant);

FIG. 8 is a photograph showing a result of analysis for interferon-α2b expression on a SDS-PAGE gel after E. coli BL21(DE3) was transformed with a pT0-IFN expression vector (lane 1: protein size marker; lane 2: pellet obtained by disrupting the IPTG-induced transformant; and lane 3: supernatants obtained by disrupting the IPTG-induced transformant);

FIG. 9 is a photograph showing a result of analysis for basic fibroblast growth factor expression on a SDS-PAGE gel after E. coli BL21(DE3) was transformed with a pT0-bFGF expression vector (lane 1: protein size marker; lane 2: pellets obtained by disrupting the IPTG-induced transformant; and lane 3: supernatants obtained by disrupting the IPTG-induced transformant);

FIG. 10 is a photograph showing a result of analysis for insulin-like growth factor-1 expression on a SDS-PAGE gel after E. coli BL21(DE3) was transformed with a pT0-IGF1 expression vector (lane 1: protein size marker; lane 2: pellet obtained by disrupting the IPTG-induced transformant; and lane 3: supernatant obtained by disrupting the IPTG-induced transformant);

FIG. 11 is a photograph showing a result of analysis for insulin-like growth factor-2 expression on a SDS-PAGE gel after E. coli BL21(DE3) was transformed with a pT0-IGF2 expression vector (lane 1: protein size marker; lane 2: pellet obtained by disrupting the IPTG-induced transformant; and lane 3: supernatant obtained by disrupting the IPTG-induced transformant);

FIG. 12 is a photograph showing a result of analysis for keratinocyte growth factor expression on a SDS-PAGE gel after E. coli BL21(DE3) was transformed with a pT0-KGF expression vector (lane 1: protein size marker; lane 2: pellets obtained by disrupting the IPTG-induced transformant; and lane 3: supernatant obtained by disrupting the IPTG-induced transformant); and

FIG. 13 is a photograph showing a result of analysis for keratinocyte growth factor expression on a SDS-PAGE gel after E. coli BL21(DE3) was transformed with a pT0N-KGF expression vector (lane 1: protein size marker; lane 2: pellets obtained by disrupting the IPTG-induced transformant; and lane 3: supernatants obtained by disrupting the IPTG-induced transformant).

BEST MODE FOR CARRYING OUT THE INVENTION

In one aspect, the present invention relates to a method of producing an active, soluble target protein in a prokaryotic cell, which is based on expressing the target protein and beta-lactamase as a first cistron and a second cistron, respectively, in a citron.

The present inventors found that, when human growth hormone expressed as inclusion bodies in prokaryotic cells and beta-lactamase were polycistronically expressed, both human growth hormone and beta-lactamase were produced in high concentrations, and the expressed human growth hormone was present in an active, soluble form. In contrast, when basic fibroblast growth factor and keratinocyte growth factor were expressed using kanamycin instead of beta-lactamase under same conditions, the target proteins were mostly expressed as inclusion bodies. Based on this finding, the present inventors polycistronically coexpressed various target proteins that are expressed in the form of inclusion bodies in prokaryotic cells but have medical usefulness, with beta-lactamase. As a result, the target proteins in this system were produced in active, soluble forms.

Thus, in another aspect, the present invention relates to a polycistronic vector to produce a heterogeneous proteins in an active, soluble form.

In an embodiment, the present invention relates to a polycistronic vector for expressing target proteins in an active, soluble form in a prokaryotic cell, which comprises (i) a promoter operable in the prokaryotic cell, (ii) a first cistron including a DNA sequence encoding the target protein, and (iii) a second cistron including a DNA sequence encoding beta-lactamase.

The term “citron”, as used herein, refers to a system where a single mRNA is synthesized from an same promoter, cistrons are separated from each other by a termination codon and an initiation codon, a ribosome binding site is present for each cistron, and proteins corresponding to each cistron are simultaneously expressed from the single mRNA transcribed in a single promoter. Herein, the “cistron” means a nucleotide sequence encoding for a single protein or polypeptide, and includes a 5′ initiation codon and a 3′ termination codon. In addition, the first and second cistrons do not mean the sequence in a DNA sequence but only indicate an individual cistron.

In a preferred aspect, in the citron of the present invention, the first cistron including a DNA sequence encoding a target protein may be in a 5′ to 3′ direction operably linked to the second cistron including a DNA sequence encoding beta-lactamase, or the second cistron including a DNA sequence encoding beta-lactamase may be in a 5′ to 3′ direction operably linked to the first cistron including a DNA sequence encoding a target protein.

The term “vector”, as used herein, refers to a DNA construct that contains a DNA sequence operably linked to a suitable regulatory sequence capable of expressing DNA in a suitable host, and, in detail, may be constructed to contain a promoter sequence, a terminator sequence, a marker gene and other suitable sequences including a suitable regulatory sequence. Such a vector may be a plasmid, a pharge, a cosmid, or the like (Molecular Cloning: Laboratory Manual second edition, Sambrook et al., Cold Spring Harbor Laboratory Press (1989)). The preparation of such a vector, mutagenesis, sequence analysis, DNA introduction into cells, gene expression and protein analysis are described in detail in Current Protocols in Molecular Biology, edited by Ausubel et al., John Wiley & Sons (1992). When introduced into a suitable host, a vector may be replicated or function independently of the host genome, or, in some cases, may be integrated into the host genome. Plasmids are at present the most common form of vectors, and, in the present invention, the terms “plasmid” and “vector” may be used interchangeably. With respect to the objects of the present invention, a vector is a vector suitable for protein expression in prokaryotic cells, and is a polycistronic vector polycistronically expressing a heterogeneous target protein and beta-lactamase.

The term “operably linked”, as used herein, means that an expression regulatory sequence is linked in such a way of regulating the transcription and translation of a polynucleotide sequence encoding a target protein, and includes maintaining a precise translation frame in such a way that a polypeptide of a target protein encoded by a polynucleotide sequence is produced when the polypeptide sequence is expressed under the control of regulatory sequences (including a promoter).

The term “promoter”, as used herein, means a minimum sequence sufficient for triggering transcription. With respect to the objects of the present invention, a promoter inducible by an external signal or an effector is used. Promoters useful for the expression of a target protein in prokaryotic cells include T7, tac, trc, lac, lpp, phoA, recA, araBAD, prou, cst-1, tetA, cada, nar, lpp-lac, starvation promoters, cspA, T7-lac operator, T3-lac operator, T5-lac operator, T4 gene 32, and nprM-lac operator. Preferred are T7, tac, lac, T7-lac operator, T3-lac operator, T5-lac operator and T4 gene 32, and more preferred are T7, tac and T7-lac operator. The most preferred promoter is T7 promoter. T7 promoter can be controlled by T7 RNA polymerase and the expression of a T7 RNA polymerase can be controlled by IPTG(isopropyl-β-D-thiogaloactosidase). T7 promoter can induce expression of a target protein in a desired time using IPTG. This is because it is preferable that a prokaryotic host cell, for example, E. coli is grown until a cell number is increased while a target protein is not expressed, and, after the E. coli cell number is sufficiently increased, the expression of the target protein is induced.

A ribosome binding site is typically located in about 10 bp upstream of an initiation codon, and functions to precisely and effectively initiate mRNA translation in polycistronic operon systems of phages or prokaryotes.

A target protein to be expressed using the polycistronic vector of the present invention may include all proteins having medical applications. In particular, proteins having demands for medical purposes but being known to be produced as inclusion bodies upon high expression in host cells by genetic engineering are suitable as the target protein of the present invention. Examples of the target protein include human growth hormone (hGH), granulocyte-colony stimulating factor (G-CSF), interferons (IFN), basic fibroblast growth factor (bFGF), insulin-like growth factor (IGF), keratinocyte growth factor (KGF), erythropoietin (EPO), thrombopoietin (TPO), human epidermal growth factor (EGF), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), nerve growth factor (NGF), transforming growth factor (TGF), tumor necrosis factor (TNF), angiogenin, angiotensin, interleukin (IL), and tissue plasminogen activator (tPA). More preferred are hGH, G-CSF, IFN-α2b, bFGF, IGF-1, IGF-2, KGF, EPO, IL-7 and TPO. These target proteins may be in a natural or modified form, and may include their variants including deletions, substitutions or additions of the whole sequence or its fragment. In another embodiment of the present invention, hGH, G-CSF, IFN-α2b, bFGF, IGF-1, IGF-2 and KGF were expressed.

The target protein to be expressed in the present invention may be present itself, or may be present in the form of a fusion protein, such as a fused form with a sequence increasing solubility, to facilitate purification, provide various functions by being fused with an antibody or enzyme, or increase solubility. When the target protein contains a sequence facilitating, for example, purification, the target protein may be expressed in the form of a fusion protein with such a sequence. Such a fused target protein may be present in a sequence of a fusion partner-a peptide linker-a target protein, but may be prepared in various organizations according to the types of target proteins and fusion partners. More preferably, the fusion partner is employed for facilitating the purification of a produced protein, and is exemplified by histidine-tag, glutathione-S-transferase, maltose-binding protein, protein A, protein G, flag peptide, thioredoxin, S-peptide, avidin, streptavidin, galactose binding protein, cellulose-binding domain, chitin-binding domain, polyarginine, polycysteine and polyphenylalanine. In a further embodiment of the present invention, a histidine tag containing ten histidines was used. The peptide linker linking the target protein to such a fusion partner includes a sequence recognizable by a proteinase, and is exemplified by enterokinase, thrombin, factor Xa, urokinase, TEV protease and subtilisin, which have a high sequence specificity.

The term “active”, as used herein, refers to a soluble protein that has biological activity by being stably expressed in a transformant with a recombinant vector and being folded into a native form without additional denaturation or refolding.

The term “soluble”, as used herein, means the nature with which a protein is not easily precipitated in an aqueous solution and does not easily form inclusion bodies or other aggregates.

The beta-lactamase (bla) used in the present invention, as a factor for selecting a host cell transformed with an expression vector, is a protein providing a resistance to ampicillin. In the polycistronic vector of the present invention, the arrangement of a cistron encoding a target protein and another cistron encoding beta-lactamase may be changed by a certain purpose, but the cistron encoding beta-lactamase is preferably located in the downstream region of the cistron encoding a target protein.

pT0 expression vectors used in embodiments of the present invention are vectors in which a fused target protein gene (fusion partner-peptide linker-target protein gene) or a target protein gene itself is operably linked to the downstream region of T7 promoter of pET3a, and which overexpresses the target protein and beta-lactamase when the fused target protein gene (or the target protein gene) and a beta-lactamase gene are transcribed under the control of an same promoter. The pT0 expression vectors prepared in the present invention include pT0191, pT0-CSF pT0-IFN, pT0-bFGF, pT0-IGF1, pT0-IGF2, pT0-KGF and pT0N-KGF, which each carry fused hGH, G-CSF, IFN-α2b, bFGF, IGF-1, IGF-2 and KGF genes, and a non-fused KGF gene. When these vectors were expressed, most of the target proteins were expressed in soluble, active forms in comparison with a control.

On the other hand, the control expression vector of pTT used in embodiments of the present invention are vectors in which a fused target protein gene (fusion partner-peptide linker-target protein gene) is operably linked to the downstream region of T7 promoter of pET3a, but in which the fused target protein gene and a beta-lactamase gene are expressed under the control of different promoters. E. coli transformed with such a vector overexpressed the fused target protein in vivo, but a large quantity of the fusion protein was expressed as inclusion bodies (Example 2). Another control vector, pTR0191, is a plasmid prepared by converting a beta-lactamase gene to a reverse orientation in a pT0191 expression vector and operably linking a fused human growth hormone gene to the downstream region of T7 promoter, and in which the target protein and the beta-lactamase gene are not located under the control of the same promoter. When the pTR0191 was expressed in a host cell, the fusion protein was mostly expressed as inclusion bodies, and beta-lactamase was expressed in low levels (Example 5).

Thus, in one detailed aspect, to simultaneously overexpress beta-lactamase and a target protein itself or a target fusion protein and express the target protein in an active form in a higher percentage, the present invention provides polycistronic vectors, pT0191, pT0-CSF, pT0-IFN, pT0-bFGF, pT0-IGF1, pT0-IGF2, pT0-KGF and pT0N-KGF, which carry respectively genes encoding hGH (SEQ ID NO. 5), G-CSF (SEQ ID NO. 7), IFN-α2b (SEQ ID NO. 9), bFGF (SEQ ID NO. 11), IGF-1 (SEQ ID NO. 13), IGF-2 (SEQ ID NO. 15) and KGF (SEQ ID NO. 23), which are fused to a pT0 expression vector derived from a pET3a expression vector, and non-fused KGF (SEQ ID NO. 25). Of them, the pT0191 and pT0-IFN are introduced into E. coli BL21(DE3), and the pT0-CSF is introduced into E. coli BL21 Star(DE3)pLysS. The resulting transformants were deposited at KCTC (Korean Collection for Type Cultures; KRIBB, 52, Oun-dong, Yusong-ku, Taejon, Korea) on Mar. 11, 2004, under accession numbers KCTC-10610BP, KCTC-10612BP and KCTC-10611BP, respectively.

The polycistronic expression vectors of the present invention may be introduced into a host cell to transform the host cell by certain methods known in the art, including chemical methods using CaCl₂ and electroporation.

The term “transformed”, as used herein, refers to introduction into a prokaryotic cell in such a manner as to allow a gene carried by the polycistronic vector to be expressed.

If a recombinant nucleotide sequence of a fusion protein is suitably transcribed to mRNA in a cell, and the cell is able to express proteins, a certain prokaryotic cell can be used, and a Gram-negative bacterium, E. coli, and a Gram-positive bacterium, Bacillus, are preferred. More preferred is E. coli, and most preferred are E. coli BL21 (DE3), E. coli BL21 Star (DE3)pLysS, E. coli HMS (DE3) and E. coli AD494 (DE3). The above host cells possess bacteriophage T₇ RNA polymerase, and the present invention is not limited to the examples. The bacteriophage-derived T7 promoter used in the present invention is more effectively expressed by bacteriophage T₇ RNA polymerase than by E. coli RNA polymerase (Studier F W et al. (1990), Method Enzymol. 185, 60-89). Thus, the pT0 expression vectors are preferably expressed by being introduced into E. coli BL21 (DE3) or E. coli BL21 Star (DE3) pLysS which carries T₇ RNA polymerase gene under control of the lacUV5 promoter. When the pT0-CSF expression vector of the present invention is introduced into E. coli BL21 (DE3), BL21 Star (DE3) pLysS, HMS (DE3) or AD494(DE3), a target fusion protein may be expressed in an active form with an efficiency of 70% or higher.

Thus, in still another aspect, the present invention provides transformants transformed with the above polycistronic expression vectors. In detail, the transformants include E. coli transformed with pT0191, pT0-CSF, pT0-IFN, pT0-bFGF, pT0-IGF1, pT0-IGF2, pT0-KGF or pT0N-KGF.

The transformants transformed with the expression vectors according to the present invention are cultured in suitable media under suitable conditions in a manner of allowing a DNA sequence encoding a target protein to be expressed. A method of expressing a recombinant protein by culturing a transformant is known in the art. For example, a transformant is inoculated in a suitable medium for seed culture, and the seed culture is inoculated in a production culture medium and grown under suitable conditions, thereby inducing protein expression. In the production culture, microbial growth is performed separately from the induction of recombinant protein expression, thereby increasing recombinant protein yield.

Thus, in still another aspect, the present invention provides a method of producing an active, soluble protein, comprising culturing the transformant and recovering a soluble target protein from a culture.

From the culture obtained by culturing a transformant, a target protein is recovered in substantially pure forms, and thus can be used for medical purposes. The recovery of a recombinant protein may be achieved by various isolation and purification methods known in the art. Typically, to remove cell debris, a cell lysate is centrifuged, and the supernatant is subjected to precipitation, dialysis and various column chromatographies. Examples of the column chromatography include ion exchange chromatography, gel-filtration chromatography, HPLC, reverse phased HPLC, preparative SDS-PAGE, and affinity column chromatography.

The purification of the soluble, active protein according to the present invention may be achieved by typical purification methods such as ultrafiltration and ion exchange chromatography without a refolding process after cell disruption and centrifugation, thereby facilitating the isolation of an active target protein.

A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as the limit of the present invention.

EXAMPLE 1 Construction of pTT191 Expression Vector

A control expression vector pTT191 was prepared to express in high yields a fusion protein containing human growth hormone as inclusion bodies.

A fusion gene encoding a fusion protein (SEQ ID NO. 5) containing human growth hormone, which is linked to a histidine-tag and an enterokinase recognition sequence, was prepared by a PCR ligation method (PCR ligation method, Willem P. C. Stemmer, and Herbert L. Heyneker (1995) Gene 164, 49-53; Scott W. Altmann, and Robert A. Kastelein (1995) Protein Expression and Purification 6, 722-726; Ana Paula de Mattos Areas, and Paulo Lee Ho (2002) Protein Expression and Purification 25, 481-487). The PCR ligation method was carried out as follows. To a PCR tube, 50 pmole of each of pairs of synthetic oligonucleotides containing twenty complementarily overlapped bases, 2.5 U (1 μl) of Pfu DNA polymerase (Stratagene, USA), 2 μL of 2.5 mM dNTPs (Takara, Japan), and 2 μl of 10×Pfu polymerase buffer were sequentially added, and sterile distilled water was added to a final volume of 20 μl. PCR was carried out using a PCR machine (MJ research, USA). Herein, each of the oligonucleotides served as a template as well as a primer. PCR conditions included denaturation at 94° C. for 5 min, and 20 cycles of denaturation at 95° C. for 1 min, annealing at 52° C. for 30 sec and elongation at 72° C. for 30 sec, followed by final elongation at 72° C. for 10 min. Two PCR products having a complementary nucleotide sequence (20 bp) at ends were amplified. To a PCR tube, 5 μl of each of the PCR products, 2.5 U (1 μl) of Pfu DNA polymerase (Stratagene, USA), 2 μl of 2.5 mM dNTPs (Takara, Japan), and 2 μl of 10×Pfu polymerase buffer were sequentially added, and sterile distilled water was added to a final volume of 20 μl. PCR was carried out using a PCR machine (MJ research, USA). PCR conditions included denaturation at 94° C. for 5 min, and 20 cycles of denaturation at 95° C. for 1 min, annealing at 52° C. for 30 sec and elongation at 72° C. for 30 sec, followed by final elongation at 72° C. for 10 min. This procedure was repeated, and, in a final step, PCR was carried with 30 cycles, thus generating a synthetic gene. The synthesized gene was run on a 1% agarose gel and isolated from the gel using QIAQuick gel extraction kit (Qiagen, USA). As a result, a fused human growth hormone (somatotropin) gene (SEQ ID NO. 5) was obtained, which included a histidine-tag (SEQ ID NO. 1) and an enterokinase recognition sequence (SEQ ID NO. 3) and an Nde I recognition sequence at both ends. Thereafter, the pTT191 expression vector was constructed as shown in FIG. 1. The synthesized fusion gene was digested with NdeI, separated on a 1% agarose gel, and isolated from the gel. The linearized fusion gene was ligated to NdeI-digested pET3a (Novagen, USA) that was also pretreated with CIAP (calf intestine alkaline phosphatase; NEB, USA). The CIAP treatment was carried out at 37° C. for one hour to prevent self-ligation of the NdeI-digested pET3a. The ligation was carried out at 16° C. for 18 hrs using T4 DNA ligase (NEB, USA), thus generating pTT191. Then, E. coli TOP10 (Invitrogen, USA) was transformed with the pTT191. The plasmid DNA was prepared from the resulting transformant and introduced into E. coli BL21 (DE3) (Novagen, USA). The resulting E. coli BL21 (DE3) transformant transformed with the pTT191 expression vector was selected on ampicillin-containing LB plates, and designated as “E. coli BL21 (DE3)/pTT191”. The correct insertion of the fusion gene containing a human growth hormone gene in the pTT191 expression vector was confirmed by digestion with restriction enzymes AlwNI and HindIII and DNA sequencing.

EXAMPLE 2 Expression of the Fusion Protein with Human Growth Hormone in the E. Coli BL21 (DE3)/pTT191 Transformant

The expression pattern of the fusion protein with human growth hormone was tested in E. coli transformed with the control expression vector pTT191.

The E. coli BL21 (DE3)/pTT191 transformed with the pTT191 expression vector was cultured in LB medium (Luria-Bertani medium) at 30° C. for 12 hrs, and the expression of the fusion protein was then induced with IPTG (Isopropyl-β-D-Thiogalactopyranoside). After IPTG induction, cells were collected by centrifugation and disrupted. After centrifugation, the supernatant was used to investigate the expression of the fusion protein. As a result, as shown in FIG. 2, the fusion protein containing human growth hormone had the predicted molecular weight of about 24 kDa, but mainly expressed as inclusion bodies.

EXAMPLE 3 Construction of pT0191 Expression Vector

A pT0191 expression vector was prepared to express the fusion protein containing human growth hormone in a soluble form in a higher percentage.

To insert a gene encoding the fusion protein with human growth hormone into a pET3a vector, PCR was carried out using pTT191 as a template, thus providing an NdeI recognition site and a HindIII recognition site to each end of a nucleotide sequence encoding the fusion protein. To a PCR tube, 100 ng of pTT191 plasmid (Example 1) as a template, 2.5 U (1 μl) of Pfu DNA polymerase (Stratagene, USA), 30 pmole of primer A (5′-AAACATATGGGCCATCATCATCATCATCATCATCATCATCAC-3′: SEQ ID NO. 19), 30 pmole of primer B (5′-AAAAAGCTTTTACTAGAAGCCACAGCTGCC-3′: SEQ ID NO. 20), 2 μl of 2.5 mM dNTPs (Takara, Japan), and 2 μl of 10×Pfu polymerase buffer were sequentially added, and sterile distilled water was added to a final volume of 20 μl. PCR was carried out using a PCR machine (MJ research, USA). PCR conditions included denaturation at 94° C. for 5 min, and 30 cycles of denaturation at 95° C. for 1 min, annealing at 58° C. for 30 sec and elongation at 72° C. for 2 min, followed by final elongation at 72° C. for 10 min. The amplified gene was digested with NdeI and HindIII restriction enzymes, separated on a 1% agarose gel and purified from the gel. The pET3a expression vector was digested with NdeI and HindIII and separated on a 1% agarose gel, and a 4119-bp fragment was purified from the gel. The NdeI/HindIII-treated fusion gene and pET3a fragment were ligated to each other at 16° C. for 18 hrs using T4 DNA ligase, thus generating pT0191. Then, E. coli TOP10 (Invitrogen, USA) was transformed with the pT0191 (FIG. 3). The plasmid DNA was prepared from the resulting transformant and introduced into E. coli BL21 (DE3). The resulting E. coli BL21 (DE3) transformant transformed with the pT0191 expression vector was selected on ampicillin-containing LB plates, and designated as “E. coli BL21 (DE3)/pT0191 (KCTC10610BP)”. The correct insertion of the fusion gene containing a human growth hormone gene in the pT0191 expression vector was confirmed by digestion with restriction enzymes NdeI and HindIII and DNA sequencing.

EXAMPLE 4 Expression of the Fusion Protein with Human Growth Hormone in the E. Coli BL21(DE3)/pT0191 Transformant

The expression pattern of the fusion protein with human growth hormone was tested in E. coli transformed with the pT0191 expression vector.

The E. coli BL21 (DE3)/pT0191 transformed with the pT0191 expression vector was cultured in LB medium at 30° C. for 12 hrs, and the expression of the fusion protein was then induced with IPTG. Thereafter, the expression of the fusion protein was estimated. As shown in FIG. 4, the fusion protein mainly expressed in an active form and was present in a centrifuged supernatant, and had a molecular weight of about 24 kDa. Unlike the E. coli BL21 (DE3)/pTT191 transformant, the E. coli BL21 (DE3)/pT0191 transformant was found to overexpress the target fusion protein along with beta-lactamase. The expression of beta-lactamase was confirmed by N-terminal sequencing.

EXAMPLE 5 Construction of pTR0191 Expression Vector and Expression of Human Growth Hormone in E. Coli BL21 (DE3)/pTR0191 Transformant

The pT0191 plasmid prepared in Example 3 was digested with SphI and HindIII, and a 3812-bp fragment was purified. A gene encoding beta-lactamase contained in the pT0191 plasmid was amplified by PCR using two primers (primer 1: 5′-AAAAAGCTTAAGGAGATGGCGCCCA-3′ (SEQ ID NO. 21); primer 2: 5′-AAAGCATGCCTAGAAGCCACAGCTG-3′ (SEQ ID NO. 22)), thus generating a 950-bp fragment in which the positions of the SphI and HindIII sites were exchanged with each other. Then, the 950-bp fragment was ligated to the 3812-bp fragment using T4 DNA ligase, thus generating a pTR0191 expression vector in which the human growth hormone gene had a different orientation from the beta-lactamase gene (FIG. 5). E. coli BL21 (DE3) was transformed with the prepared expression vector, and protein expression was carried out at 30° C. As shown in FIG. 6, the target protein was expressed mainly as inclusion bodies, and the beta-lactamase was expressed in lower levels than the case of using the pT0191 expression vector.

EXAMPLE 6 Construction of pT0-CSF Expression Vector

A pT0-CSF expression vector was prepared to express in high yields a fusion protein containing human granulocyte-colony stimulating factor (G-CSF) in a soluble form. A gene (SEQ ID NO. 7) encoding a fusion protein containing human G-CSF linked to a histidine-tag and an enterokinase recognition sequence was synthesized according to the same PCR ligation method as in Example 1, and the pT0-CSF expression vector was constructed according to the same method as in Example 3 (FIG. 3). The pT0-CSF expression vector was introduced into E. coli BL21 Star(DE3) pLysS (Invitrogen, USA), and the resulting transformant was designated as “E. coli BL21 Star(DE3)pLysS/pT0-CSF (KCTC10611BP)”. The correct insertion of the fusion gene containing a human G-CSF gene into the pT0-CSF expression vector was confirmed by digestion with NdeI and HindIII and DNA sequencing.

EXAMPLE 7 Expression of the Fusion Protein with Human G-CSF in the E. coli B21 Star(DE3)pLysS/pT0-CSF Transformant

The expression pattern of the fusion protein with human G-CSF was investigated in E. coli transformed with the pT0-CSF expression vector.

The E. coli B21 Star(DE3)pLysS/pT0-CSF transformed with the pT0-CSF expression vector was cultured in LB medium at 30° C. for 12 hrs, and the expression of the fusion protein was then induced with IPTG. As shown in FIG. 7, the fusion protein containing human G-CSF was expressed mainly in an active form and present in a centrifugal supernatant, and had a molecular weight of about 20 kDa. Like the E. coli BL21 (DE3)/pT0191 transformant, the E. coli B21 Star(DE3)pLysS/pT0-CSF transformant transformed with the pT0-CSF expression vector was found to overexpress the target fusion protein containing human G-CSF along with beta-lactamase.

In addition, the pT0-CSF expression vector was introduced into E. coli BL21 (DE3) (Novagen, USA), E. coli HMS (DE3) (Novagen, USA) and E. coli AD494 (DE3) (Novagen, USA) to investigate the expression pattern of the fusion protein containing human G-CSF (also designated herein as human G-CSF fusion protein) in the E. coli strains. The results are given in Table 1, below. As apparent from the data of Table 1, the fusion protein was mainly expressed in an active form.

TABLE 1 Volume (arbitary unit [a.u.]) Active Inclusion Percentage of active Host cell (E.coli) Total form bodies form* BL21 (DE3) 15,043 10,610 4,433 70.5% BL21 Star (DE3) 28,169 25,554 2,614 90.7% pLysS HMS (DE3) 13,376 12,361 1,015 92.4% AD 494 (DE3) 22,843 16,886 5,957 73.9% *measured by a densitometer after electorphoresis

EXAMPLE 8 Construction of pT0-IFN, pT0-bFGF, pT0-IGF1, pT0-IGF2, pT0-KGF and pT0N-KGF Expression Vectors

Several expression vectors, pT0-IFN, pT0-bFGF, pT0-IGF1, pT0-IGF2, pT0-KGF and pT0N-KGF, were prepared to express in high yields various target proteins themselves or fusion proteins containing target proteins in soluble forms. Genes coding for an IFN-α2b fusion protein (SEQ ID NO. 9), a bFGF fusion protein (SEQ ID NO. 11), an IGF-1 fusion protein (SEQ ID NO. 13), an IGF-2 fusion protein (SEQ ID NO. 15) and a KGF fusion protein (SEQ ID NO. 23), which each are linked to a histidine-tag and an enterokinase recognition sequence, and a non-fused KGF itself (SEQ ID NO. 25) were prepared according to the same PCR ligation method as in Example 1, wherein primers were designed to provide a NdeI site and a HindIII site at each end of a sequence of each of the coding genes to insert fusion protein into pET3a vector. The amplified genes were digested with NdeI and HindIII, separated on a 1% agarose gel, and purified from the gel. A pET3a expression vector was digested with NdeI and HindIII and separated on a 1% agarose gel, and a 4119-bp fragment was purified from the gel. Each of the NdeI/HindIII-treated fused genes and pET3a fragment was ligated to each other at 16° C. for 18 hrs using T4 DNA ligase, thus generating expression vectors, pT0-IFN, pT0-bFGF, pT0-IGF1, pT0-IGF2, pT0-KGF and pT0N-KGF, respectively. Then, E. coli TOP10 was transformed with the expression vectors. The plasmid DNA was prepared from each of the resulting transformants and introduced into E. coli BL21 (DE3). The correct insertion of each gene into the corresponding expression vector was confirmed by digestion with NdeI and HindIII and DNA sequencing.

EXAMPLE 9 Expression of Each Target Protein in E. Coli BL21 (DE3)/pT0-IFN, pT0-bFGF, pT0-IGF1, pT0-IGF2, pT0-KGF and pT0N-KGF Transformants

The expression pattern of each target protein was investigated in E. coli transformed with the expression vectors, pT0-IFN, pT0-bFGF, pT0-IGF1, pT0-IGF2, pT0-KGF and pT0N-KGF.

The E. coli BL21 (DE3) transformants transformed with the above expression vectors were individually cultured in LB medium at 30° C. for 12 hrs, and the expression of each target protein was then induced with IPTG. The results are given in FIGS. 8 to 13. As shown in the figures, the expressed target proteins were present in a centrifugal supernatant, thus indicating that the target proteins are expressed in soluble, active forms.

INDUSTRIAL APPLICABILITY

As described hereinbefore, the present invention provides expression vectors overexpressing a target protein along with beta-lactamase. The expression vectors can produce in high levels a soluble, active form of heterogeneous target proteins in prokaryotic cells where the proteins are mostly expressed as inclusion bodies when other expression vectors are used. 

1. A method of producing an active, soluble target protein in a prokaryotic cell, comprising expressing the target protein and beta-lactamase as a first cistron and a second cistron, respectively, in a polycistron, wherein the polycistron is derived from a recombinant vector comprising (i) a promoter operable in the prokaryotic cell, (ii) a first cistron including a DNA sequence encoding the target protein and (iii) a second cistron including a DNA sequence encoding the beta-lactamase under conditions such that the target protein is produced in an active, soluble form.
 2. The method according to claim 1, wherein the polycistron comprises in an operable manner in a 5′ to 3′ direction (i) a promoter operable in the prokaryotic cell, (ii) a first cistron including a DNA sequence encoding the target protein and (iii) a second cistron including a DNA sequence encoding beta-lactamase.
 3. The method according to claim 1, wherein the polycistron is derived from from a recombinant vector comprising in an operable manner in a 5′ to 3′ direction (i) a promoter operable in the prokaryotic cell, (ii) a second cistron including a DNA sequence encoding beta-lactamase and (iii) a first cistron including a DNA sequence encoding the target protein.
 4. The method according to claim 1, wherein the target protein is selected from the group consisting of human growth hormone, granulocyte-colony stimulating factor, interferon, basic fibroblast growth factor, insulin-like growth factor, keratinocyte growth factor, erythropoietin, thrombopoietin, human epidermal growth factor, platelet-derived growth factor, vascular endothelial growth factor, nerve growth factor, transforming growth factor, tumor necrosis factor, angiogenin, angiotensin and interleukin.
 5. The method according to claim 4, wherein the target protein is selected from the group consisting of human growth hormone, granulocyte-colony stimulating factor, interferon-α2b, basic fibroblast growth factor, insulin-like growth factor-1, insulin-like growth factor-2 and keratinocyte growth factor.
 6. The method according to claim 1, wherein the prokaryotic cell is Escherichia coli (E. coli).
 7. The method according to claim 6, wherein the E. coli is selected from the group consisting of E. coli BL21 (DE3), E. coli BL21 Star (DE3) pLys S, E. coli HMS (DE3) and E. coli AD494 (DE3).
 8. A polycistronic vector for expressing a target protein in an active, soluble form in a prokaryotic cell, comprising: (i) a promoter operable in the prokaryotic cell; (ii) a first cistron including a DNA sequence encoding the target protein; and (iii) a second cistron including a DNA sequence encoding beta-lactamase.
 9. The polycistronic vector according to claim 8, wherein the vector comprises in an operable manner in a 5′ to 3′ direction (i) a promoter operable in the prokaryotic cell, (ii) a first cistron including a DNA sequence encoding the target protein and (iii) a second cistron including a DNA sequence encoding beta-lactamase.
 10. The polycistronic vector according to claim 8, wherein the vector comprises in an operable manner in a 5′ to 3′ direction (i) a promoter operable in the prokaryotic cell, (ii) a second cistron including a DNA sequence encoding beta-lactamase and (iii) a first cistron including a DNA sequence encoding the target protein.
 11. The polycistronic vector according to claim 8, wherein the target protein is selected from the group consisting of human growth hormone, granulocyte-colony stimulating factor, interferon, basic fibroblast growth factor, insulin-like growth factor, keratinocyte growth factor, erythropoietin, thrombopoietin, human epidermal growth factor, platelet-derived growth factor, vascular endothelial growth factor, nerve growth factor, transforming growth factor, tumor necrosis factor, angiogenin, angiotensin and interleukin.
 12. The polycistronic vector according to claim 11, wherein the target protein is selected from the group consisting of human growth hormone, granulocyte-colony stimulating factor, interferon-α2b, basic fibroblast growth factor, insulin-like growth factor-1, insulin-like growth factor-2 and keratinocyte growth factor.
 13. The polycistronic vector according to claim 8, wherein the promoter is selected from the group consisting of T7, tac, trc, lac, lpp, phoA, recA, araBAD, proU, cst-1, tetA, cadA, nar, lpp-lac, starvation promoters, cspA, T7-lac operator, T3-lac operator, T5-lac operator, T4 gene 32 and nprM-lac operator.
 14. The polycistronic vector according to claim 13, wherein the promoter is T7 promoter.
 15. The polycistronic vector according to claim 8, wherein the vector is selected from the group consisting of pT0191, pT0-CSF, pT0-IFN, pT0-bFGF, pT0-IGF1, pT0-IGF2, pT0-KGF and pT0N-KFG.
 16. A transformant transformed with the expression vector of claim
 8. 17. The transformant according to claim 16, wherein the transformant is Escherichia coli (E. coli).
 18. A method of producing an active, soluble target protein, comprising culturing the transformant of claim 17 and recovering an expressed target protein from a culture. 